As to a treatment technique of the exhaust gas generated from an internal-combustion engine, many techniques have been proposed conventionally. In particular, as to an exhaust gas treatment from a diesel engine, various techniques have been proposed for the purpose of reduction of exhaust of the particulate matter (PM) and NOx contained in exhaust gas. For example, as a catalyst for purifying exhaust gas, an oxidation catalyst that oxidizes a carbon monoxide (hereinafter, also referred to as “CO”) and a hydrocarbon (hereinafter, also referred to as “HC”) to CO2 or H2O, a NOx storage catalyst that stores a nitrogen oxide (hereinafter, also referred to as “NOx”), further, a three way catalyst that removes NOx, CO, and HC at the same time, and the like have been proposed.
In recent years, due to the strengthening of the regulations on exhaust gas, a CO treatment technique that is efficient at a low temperature of exhaust gas is desired, and for example, a technique that improves CO treatment efficiency at a low temperature by using an alumina, an oxide such as zirconia or ceria, having a fine particle diameter, and a noble metal such as platinum in combination has been proposed (Patent Literature 1). As another technique, a technique that can effectively treat CO by a noble metal, and a complex oxide of aluminum and zirconium or a complex oxide of aluminum, zirconium and titanium even at a low exhaust gas temperature has been proposed (Patent Literature 2). It has been disclosed that the noble metal is preferably palladium having a nano-order particle diameter, and the smaller the particle of palladium is, the better performance of the CO purification at a low temperature it get.